Switch chassis with flexible topology

ABSTRACT

A method for communication includes providing multiple chassis. Each chassis includes a plurality of slots, which are arranged in at least an outer tier and a middle tier, and which are configured for insertion therein of respective switches. In at least a first chassis among the multiple chassis, first internal interconnects are connected between the slots in the middle tier and the slots in the outer tier, so as to connect each of the slots in the middle tier to multiple slots in the outer tier. In at least a second chassis among the multiple chassis, second internal interconnects are connected directly between the slots in the outer tier. External interconnects are connected between at least some of the slots in the outer tier of the first chassis and at least some of the slots in the outer tier of the second chassis in order to define a network.

FIELD OF THE INVENTION

The present invention relates generally to data communication networks,and particularly to apparatus and methods for switching in suchnetworks.

BACKGROUND

Various types of switching networks are known in the art. Some of thesetypes have hierarchical topologies, meaning that the switches in thenetwork are connected together in multiple stages (or levels). A signalinput at one of the edge ports of the switching network, at the bottomof the hierarchy, generally pass through one or more internal stages, athigher levels in the hierarchy, before reaching another edge port fromwhich it is output. Some hierarchical networks are non-blocking, in thesense that any unused edge port can always be connected to any otherunused edge port, regardless of existing connections within the network.

A fat-tree network is a kind of hierarchical switching network, alsoreferred to as a CBB or Clos network, which is made up of crossbarswitches arranged in interconnected stages. Fat-tree networks wereoriginally defined by Leiserson in “Fat-Trees: Universal Networks forHardware Efficient Supercomputing,” IEEE Transactions on ComputersC-34(10), pages 892-901 (1985), and are now widely used in parallel anddistributed computing architectures. Further aspects of fat trees aredescribed, for example, in U.S. Pat. No. 8,570,865, whose disclosure isincorporated herein by reference.

In a fat-tree network, computing nodes are connected to leaf switches atthe edge of the network, and the leaf switches are interconnected bylinks to spine switches arranged in multiple hierarchical levels insidethe network. The numbers of spine switches and links at each level arechosen so as to provide sufficient bandwidth between the spine switchesto enable multiple computing nodes (or all of the computing nodes) toexchange messages via the network concurrently. The term “fat-tree” isused broadly in the art to define any and all types of communicationnetworks meeting the above criteria and includes extended andgeneralized types of fat trees.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide apparatus and methods for assembly of switching networks withenhanced flexibility.

There is therefore provided, in accordance with an embodiment of theinvention, switching apparatus, which includes a chassis, including aplurality of slots, which are arranged in at least three tiers,including first and second outer tiers and a middle tier, and which areconfigured for insertion therein of respective switches, each switchhaving multiple ports. First and second internal interconnects extendbetween the slots in the middle tier and the slots in the first andsecond outer tiers, respectively, so as to connect each of the slots inthe middle tier to multiple slots in each of the first and second outertiers. External interconnects extend from the slots in the outer tiersfor connection to components outside the apparatus. Jumper modules areconfigured for insertion in the slots in the middle tier in place of therespective switches. Each jumper module includes multiple jumpers forconnecting the first internal interconnects directly to respective onesof the second internal interconnects.

In some embodiments, each slot has multiple terminals, which connect tothe ports of a switch when the switch is inserted into the slot, and theinterconnects include conductors extending between the terminals of theslots. Typically, each of the jumpers in the jumper modules isconfigured to create a short-circuit between a pair of the terminalswhen the jumper modules are inserted into the slots in the middle tier.In a disclosed embodiment, the chassis includes a backplane, theswitches are configured as circuit boards, and the slots includereceptacles that are configured to receive the circuit boards.Alternatively, the chassis includes a circuit board, and the switchesare contained in integrated circuit packages, and the slots includesockets that are configured to receive the integrated circuit packages.

In a disclosed embodiment, the first and second internal interconnectsconnect each of the slots in the middle tier to all of the slots in eachof the first and second outer tiers, so that when a switch is insertedinto any given slot in the middle tier, each of the slots in each of theouter tiers is connected to a respective one of the ports of the switch.Typically, the jumper modules are configured so that when a jumpermodule is inserted into the given slot in the middle tier, each of theslots in the first outer tier is connected by at least one of thejumpers in the jumper module to a respective one of the slots in thesecond outer tier.

There is also provided, in accordance with an embodiment of theinvention, a switching network, including a first switching apparatus,as described above, in which all of the slots are populated with theswitches. In multiple second switching apparatuses of the sort describedabove, the slots in the middle tier are populated with the jumpermodules, and the external interconnects of the slots in the first outertier are connected to respective ones of the external interconnects ofthe slots in the outer tiers of the first switching apparatus.

In one embodiment, the first and second switching apparatuses areconfigured and interconnected so as to define a fat tree.

Additionally or alternatively, the external interconnects of the slotsin the second outer tier of the second switching apparatuses are coupledto other components of the network, selected from a set of componentsconsisting of other switching apparatuses and computing nodes.

There is additionally provided, in accordance with an embodiment of theinvention, a method for communication, which includes providing multiplechassis. Each chassis including a plurality of slots, which are arrangedin at least three tiers, including first and second outer tiers and amiddle tier, and which are configured for insertion therein ofrespective switches, each switch having multiple ports. First and secondinternal interconnects extend between the slots in the middle tier andthe slots in the first and second outer tiers, respectively, so as toconnect each of the slots in the middle tier to multiple slots in eachof the first and second outer tiers. External interconnects extend fromthe slots in the outer tiers for connection to components outside thechassis. Jumper modules are inserted in the slots in the middle tier inplace of the respective switches in one or more of the chassis. Eachjumper module includes multiple jumpers for connecting the firstinternal interconnects directly to respective ones of the secondinternal interconnects. At least some of the external interconnects ofthe multiple chassis are coupled together in order to define a network.

There is further provided, in accordance with an embodiment of theinvention, a method for communication, which includes providing multiplechassis, each chassis including a plurality of slots, which are arrangedin at least an outer tier and a middle tier, and which are configuredfor insertion therein of respective switches, each switch havingmultiple ports. In at least a first chassis among the multiple chassis,first internal interconnects are connected between the slots in themiddle tier and the slots in the outer tier, so as to connect each ofthe slots in the middle tier to multiple slots in the outer tier. In atleast a second chassis among the multiple chassis, second internalinterconnects are connected directly between the slots in the outertier. External interconnects are connected between at least some of theslots in the outer tier of the first chassis and at least some of theslots in the outer tier of the second chassis in order to define anetwork.

In one embodiment, connecting the second internal interconnects includesinserting jumper modules in the slots in the middle tier of the secondchassis in place of the respective switches. In another embodiment, theinternal and external interconnects include cables.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is block diagram that schematically illustrates a switch chassis,in accordance with an embodiment of the invention;

FIGS. 2A and 2B are block diagrams that schematically illustrate jumpermodules, in accordance with an embodiment of the invention;

FIG. 3 is a block diagram that schematically illustrates a switchchassis in a fully-populated configuration, in accordance with anembodiment of the invention;

FIG. 4 is a block diagram that schematically illustrates a switchchassis in a partly-populated configuration, in accordance with anembodiment of the invention; and

FIG. 5 is a block diagram that schematically shows a fat-tree networkassembled from the switch chassis of FIGS. 4 and 5, in accordance withan embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In many large-scale, high-speed computing networks, such as data centernetworks, the computing nodes (servers) are interconnected by a fat-treenetwork containing a large number of switches, in a hierarchy ofmultiple tiers. It is not practical to mount all of these switches in asingle chassis. Therefore, the network is typically built up frommultiple chassis, each containing one or more switches, either as astand-alone switches or as part of chassis, which may or may not bemanaged as a single switch. The chassis themselves are also arrangedhierarchically, with one or more chassis holding as the highest-levelspines, and other chassis containing middle-level spines and leafswitches. The internal building blocks and topologies of the chassis mayvary depending upon their location in the hierarchy. Therefore, vendorsof switching components typically supply two or more different types ofchassis, which the system integrator must select, stock, and connectproperly in order to achieve the desired network configuration. Thisapproach may lead to inefficient use of building blocks and topologiesin order to support a variety of use cases with a single or limitednumber of designs.

Embodiments of the present invention that are described herein providechassis with a flexible internal topology, so that the same chassis canbe used for multiple functions and deployed at multiple positions withina hierarchical switching network. Specifically, as described in detailhereinbelow, these chassis can configured, for example by means ofinternal jumpers, to serve at all levels of a fat-tree network with highefficiency. The consequent reduction in the number of different chassistypes and ancillary components that the vendor must supply and thesystem integrator must maintain leads to a reduction in the capitalcosts and operating costs of the network.

The disclosed embodiments provide chassis for switching apparatus,having multiple slots into which switches can be inserted. The term“switch” refers to any electronic component having multiple externalports and internal logic for selectably interconnecting the ports. Thus,a switch, in the context of the present description and in the claims,may range from a simple crossbar switch to a variety of multi-portdevices with forwarding, routing, and/or admission control capabilities,such as a bridge, router or suitable network processor.

The term “chassis” is used broadly, in the context of the presentdescription and in the claims, to refer to any sort of plug-in frameworkfor switch components, wherein the slots of the chassis have terminalsthat connect to the ports of the switch that is inserted into the slot.For example, when using switches configured as circuit boards, thechassis typically comprises a backplane, with receptacles for thecircuit boards. Alternatively, for switches contained in integratedcircuit (IC) packages, the slots comprise appropriate sockets for the ICpackages, while the chassis may comprise a printed circuit board onwhich the sockets are mounted. Alternatively, the principles of thepresent invention may be implemented using chassis of other sorts.

In the disclosed embodiments, the slots in each chassis are arranged inat least three tiers, including two outer tiers and a middle tierbetween them. Each of the slots in the middle tier is connected tomultiple slots in the outer tiers (or, in some embodiments, to all ofthe slots in the outer tiers) by respective internal interconnects. Inaddition, external interconnects extend from the slots in the outertiers for connection to other components outside the chassis, such asother switching apparatuses or computing nodes, for example. The twoouter tiers are topologically identical, in the sense that all the slotsin these tiers have both internal interconnects to the middle tier andexternal interconnects. It is therefore also possible to regard the twoouter tiers in the present description simply as different partitions ofa single outer tier; and the functionality and connectivity of thechassis provided by embodiments of the present invention do not requirethat the two outer tiers be physically separated, for example bypositioning them on opposite sides of the middle tier. Rather, the tiersare defined and differentiated on the basis of their connectivity withinand outside the chassis.

The chassis in the switching apparatus has (at least) twoconfigurations, which depend upon whether switches or jumper modules areinserted in the slots in the middle tier. Each jumper module comprisesmultiple jumpers, which create short-circuits between pairs of theterminals in the slot in which it is inserted, thus connecting the slotsin the outer tiers directly to one another via the internalinterconnects of the chassis. (The term “directly connected,” in thecontext of the present description and in the claims, means that theterminals in question are connected to one another without anintervening switch.) When the middle tier of the chassis is populatedwith switches, the apparatus is suitable for deployment at the highestlevel of a fat-tree network, for example. On the other hand, with jumpermodules in the middle tier, the chassis is suitable for use in a middleor leaf level of the network. Because the chassis contains a smallernumber of switches in this latter configuration, the power consumptionand cost of the apparatus are reduced. At the same time, the entirenetwork can be built up, as explained above, using multiple chassis ofthe same type.

Although the disclosed embodiments relate to certain particular networktypes and topologies, the principles of the present invention aresimilarly applicable to other sorts of large-scale switching networks,as will be apparent to those skilled in the art. Furthermore, theseprinciples may be implemented not only in conventional chassistechnologies (such as hard-wired backplanes and printed circuit boards),but also using flexibly-configured backplanes, with connections made bycables and/or switches integrated within the backplane. For example, ina cable-based backplane, the two (or more) different configurations ofthe chassis can be produced by connecting cables as internalinterconnects between the slots in the middle tier and the slots in theouter tier in one chassis configuration, while connecting cablesdirectly between different slots in the outer tier in another chassisconfiguration. Although jumper modules offer a particularly simple,reliable and convenient means for modifying the internal connectivity ofthe backplane, such modules are not required in all embodiments.

FIG. 1 is block diagram that schematically illustrates a switch chassis20, in accordance with an embodiment of the invention. As noted earlier,chassis 20 typically comprises a backplane or other printed circuitboard containing slots 22 and 24 in the form, for example, of suitableedge connector receptacles or IC sockets, as are known in the art. Eachslot is configured for insertion of a switch therein, as shown in thefigures that follow, and contains terminals (not shown) that connect tothe ports of the switch when the switch is inserted into the socket.

The slots in chassis 20 are arranged in three tiers: slots 22 inopposing outer tiers and slots 24 in a middle tier between the outertiers. (As noted above, the two outer tiers shown in FIG. 1 can beviewed as partitions of a single tier, as illustrated particularly bythe configuration of apparatuses 40 that is illustrated in FIG. 5 anddescribed below.) Internally, slots 22 and 24 can be identical, so thatthe same switch can be inserted into any of them. The slots differ inthat external interconnects 26 extend from slots 22 in the outer tiersfor connection to components outside chassis 20. Internal interconnects28 connect each of slots 24 in the middle tier to slots 22 in the outertiers. Typically (although not necessarily), internal interconnects 28connect each of slots 24 in the middle tier to all of slots 22 in eachof the two outer tiers. Thus, when a switch is inserted into any givenslot 24 in the middle tier, each of slots 22 in each of the outer tiersis connected to a respective port of the switch. As a result, thetopology of chassis is non-blocking, and any of the switches in a givenouter tier can communicate with all of the switches in the other outertier through each of the switches in the middle tier.

For the sake of simplicity, chassis 20 is shown in the figures ascontaining only two sockets 22, 24 in each tier, with four interconnectsper socket, corresponding to four ports per switch. In practice, chassis20 typically contains a larger number of sockets in each tier, forexample, four, eight, or even more sockets, supporting eight or moreports per switch. Furthermore, the principles of the present inventionmay readily be extended to chassis containing four or more tiers ofsockets, containing two or more middle tiers, rather than a singlemiddle tier as in the pictured embodiments, as well as chassis withsockets having different numbers of interconnects.

FIGS. 2A and 2B are block diagrams that schematically illustrate jumpermodules 30 and 38, respectively, in accordance with an embodiment of theinvention. These jumper modules are configured for insertion in slots 24in the middle tier of chassis 20, in place of the switches. The jumpermodules have terminals 32, 34 in place of the switch ports forconnection, via the slot terminals, to internal interconnects 28. Jumpermodules 30 and 38 are made to be physically and electrically compatiblewith the corresponding switches, so that they can conveniently beplugged into and out of slots 24 in order to change the configuration(i.e., the internal topology) of the chassis. Thus, for example, modules30 and 38 may comprise substrates such as printed circuit boards withedge connectors identical to those of the switch boards, or interposersconfigured to plug into the same sockets as the IC packages containingthe switches. Alternatively, the functionality of these jumper modulesmay be implemented using any suitable sort of interconnects that areknown in the art.

Each jumper module comprises multiple jumpers 36 extending betweenterminals 32 and 34, for connecting the internal interconnects 28 thatlink the given socket 24 to sockets 22 in one of the outer tiers to theinternal interconnects 28 that link this socket 24 to sockets 22 in theother outer tier. Jumpers 36 typically comprise either wires or printedconductors, which create short-circuits between the terminals of slot 24to which terminals 32 and 34 are connected. In module 30, jumpers 36connect terminal 32 on one side of the module to terminal 32 on theother, and similarly connect terminals 34 one to the other. In module38, on the other hand, jumpers 36 cross-connect the terminals. (Thereason for using these different module configurations is illustrated inFIG. 4.) Although only two types of jumper modules are shown in thepresent simplified example, chassis with larger numbers of slots mayrequire a larger number of different jumper modules. In any case, thejumper modules remain simple, inexpensive components.

FIG. 3 is a block diagram that schematically illustrates switchingapparatus 40, comprising switch chassis 20 in a fully-populatedconfiguration, in accordance with an embodiment of the invention.Switches 42 are plugged into all of the slots of the chassis, withswitches labeled SW0, SW1, SW2 and SW3 in the outer tiers and switchesSW4 and SW5 in the middle tier. Each of the switches in the outer tiersis connected to a respective port on SW4 and a respective port on SW5.

FIG. 4 is a block diagram that schematically illustrates switchingapparatus 44, comprising switch chassis 20 in a partly-populatedconfiguration, in accordance with another embodiment of the invention.In this case, switches SW4 and SW5 in the middle tier are replaced byjumper modules 30 and 38, respectively. When the jumper modules areinserted into the slots in the middle tier in this manner, each of theslots in each outer tier is connected by one of jumpers 36 in eachjumper module 30, to a respective slot in the other outer tier.Alternatively, when sockets 24 have a larger number of internalinterconnects 28, each of the slots in each of the outer tiers may beconnected by two or more jumpers to the respective slot in the otherouter tier.

Specifically, in the present example, each port of switches 42 in eachof the outer tiers is connected via jumper module 30 or 38 to acorresponding port of a switch in the other outer tier, and vice versa.In other words, apparatus 44 operates as a two-tier switch chassis, witheach switch in each tier connected to all of the switches in the othertier. This same sort of connectivity could be achieved in apparatus 40(FIG. 3), by static configuration of switches 42 in the middle tier (SW4and SW5) to give the connectivity of jumper modules 30 and 38. Thisalternative, however, suffers from the drawbacks of higher powerconsumption and the wasted extra cost of two unneeded switches in themiddle tier.

FIG. 5 is a block diagram that schematically shows a fat-tree network 50assembled from apparatuses 40 and 44 (as shown in FIGS. 4 and 5), inaccordance with an embodiment of the invention. Two apparatuses 40, inwhich all slots of the chassis are populated with switches 42, serve asthe highest-level spine, with switches SW4 and SW5 at the top of thehierarchy. Multiple apparatuses 44 (with the jumper modules in the slotsin the middle tier omitted from this figure for the sake of simplicity)serve as middle-level spines or leaves. Each external interconnect 26 ofswitches SW2 and SW3, in the upper outer tier of each of apparatuses 44,is connected to a respective external interconnect of one of switchesSW0, SW1, SW2 and SW3 in the outer tiers of apparatus 40. Externalinterconnects of switches SW0 and SW1, in the lower outer tier ofapparatuses 44, are connected either to switches in other, similarapparatuses 44 in the next level down of network 50 or to computingnodes (not shown).

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

The invention claimed is:
 1. Switching apparatus, comprising: a chassis,comprising a plurality of slots, which are arranged in at least threetiers, comprising first and second outer tiers and a middle tier, andwhich are configured for insertion therein of respective switches, eachswitch having multiple ports; first and second internal interconnectsextending between the slots in the middle tier and the slots in thefirst and second outer tiers, respectively, so as to connect each of theslots in the middle tier to multiple slots in each of the first andsecond outer tiers; external interconnects extending from the slots inthe outer tiers for connection to components outside the apparatus; andjumper modules configured for insertion in the slots in the middle tierin place of the respective switches, each jumper module comprisingmultiple jumpers for connecting the first internal interconnectsdirectly to respective ones of the second internal interconnects.
 2. Theapparatus according to claim 1, wherein each slot has multipleterminals, which connect to the ports of a switch when the switch isinserted into the slot, wherein the interconnects comprise conductorsextending between the terminals of the slots, wherein each one of thejumper modules comprises a plurality of jumper-module terminalsconfigured to connect to the terminals of the slots in the middle tierin place of the ports of the switches, and wherein the jumpers connectthose of the jumper-module terminals that are on one side of the jumpermodule to those of the jumper-module terminals that are on another sideof the jumper module.
 3. The apparatus according to claim 2, whereineach of the jumpers in the jumper modules is configured to create ashort-circuit between a pair of the terminals when the jumper modulesare inserted into the slots in the middle tier.
 4. The apparatusaccording to claim 2, wherein the chassis comprises a backplane, theswitches are configured as circuit boards, the slots comprisereceptacles that are configured to receive the circuit boards, thejumper modules comprise printed circuit boards configured to plug intothe receptacles in place of the switches, and the jumpers compriseconductors printed onto the printed circuit boards.
 5. The apparatusaccording to claim 4, wherein the printed circuit boards comprise edgeconnectors identical to edge connectors of the switches.
 6. Theapparatus according to claim 2, wherein the chassis comprises a circuitboard, the switches are contained in integrated circuit packages, theslots comprise sockets that are configured to receive the integratedcircuit packages, and the jumper modules comprise interposers configuredto plug into the sockets in place of the integrated circuit packages. 7.The apparatus according to claim 6, wherein the jumper modules areconfigured so that when a jumper module is inserted into the given slotin the middle tier, each of the slots in the first outer tier isconnected by at least one of the jumpers in the jumper module to arespective one of the slots in the second outer tier.
 8. The apparatusaccording to claim 1, wherein the first and second internalinterconnects connect each of the slots in the middle tier to all of theslots in each of the first and second outer tiers, so that when a switchis inserted into any given slot in the middle tier, each of the slots ineach of the outer tiers is connected to a respective one of the ports ofthe switch.
 9. A switching network, comprising: multiple chassis of asame type, including at least a first chassis and a plurality of secondchassis, each of the chassis comprising a plurality of slots, which arearranged in at least three tiers, comprising first and second outertiers and a middle tier, and having external interconnects extendingfrom the slots in the outer tiers and first and second internalinterconnects extending between the slots in the middle tier and theslots in the first and second outer tiers, respectively, so as toconnect each of the slots in the middle tier to multiple slots in eachof the first and second outer tiers; first switches inserted into all ofthe slots in the first chassis; second switches inserted into the slotsin the outer tiers of each of the plurality of the second chassis; andjumper modules inserted into the slots in the middle tier of each of theplurality of the second chassis, each jumper module comprising multiplejumpers for connecting the first internal interconnects directly torespective ones of the second internal interconnects, wherein theexternal interconnects of the slots in the first outer tier of each ofthe second chassis are connected to respective ones of the externalinterconnects of the slots in the outer tiers of the first chassis. 10.The network according to claim 9, wherein the first and second chassisare configured and interconnected so as to define a fat tree.
 11. Thenetwork according to claim 9, wherein the external interconnects of theslots in the second outer tier of the second chassis are coupled toother components of the network, selected from a set of componentsconsisting of other switching apparatuses and computing nodes.
 12. Amethod for communication, comprising: providing multiple chassis, eachchassis comprising a plurality of slots, which are arranged in at leastthree tiers, comprising first and second outer tiers and a middle tier,and which are configured for insertion therein of respective switches,each switch having multiple ports, with first and second internalinterconnects extending between the slots in the middle tier and theslots in the first and second outer tiers, respectively, so as toconnect each of the slots in the middle tier to multiple slots in eachof the first and second outer tiers, and external interconnectsextending from the slots in the outer tiers for connection to componentsoutside the chassis; inserting jumper modules in the slots in the middletier in place of the respective switches in one or more of the chassis,each jumper module comprising multiple jumpers for connecting the firstinternal interconnects directly to respective ones of the secondinternal interconnects; and coupling together at least some of theexternal interconnects of the multiple chassis in order to define anetwork.
 13. The method according to claim 12, wherein each slot hasmultiple terminals, which connect to the ports of a switch when theswitch is inserted into the slot, and wherein the interconnects compriseconductors extending between the terminals of the slots.
 14. The methodaccording to claim 13, wherein inserting the jumper modules comprisescreating, with each of jumpers, a short-circuit between a pair of theterminals when the jumper modules are inserted into the slots in themiddle tier.
 15. The method according to claim 13, wherein the chassiscomprises a backplane, the switches are configured as circuit boards,and the slots comprise receptacles that are configured to receive thecircuit boards.
 16. The method according to claim 13, wherein thechassis comprises a circuit board, and the switches are contained inintegrated circuit packages, and the slots comprise sockets that areconfigured to receive the integrated circuit packages.
 17. The methodaccording to claim 12, wherein the first and second internalinterconnects connect each of the slots in the middle tier to all of theslots in each of the first and second outer tiers, so that when a switchis inserted into any given slot in the middle tier, each of the slots ineach of the outer tiers is connected to a respective one of the ports ofthe switch.
 18. The method according to claim 17, wherein inserting thejumper modules comprises, when a jumper module is inserted into thegiven slot in the middle tier, connecting each of the slots in the firstouter tier by at least one of the jumpers in the jumper module to arespective one of the slots in the second outer tier.
 19. The methodaccording to claim 12, wherein coupling together the at least some ofthe external interconnects comprises coupling a first chassis, in whichall of the slots are populated with the switches, to multiple secondchassis, in which the slots in the middle tier are populated with thejumper modules, such that the external interconnects of the slots in thefirst outer tier of the multiple second chassis are connected torespective ones of the external interconnects of the slots in the outertiers of the first chassis.
 20. The method according to claim 19,wherein coupling the first chassis to the multiple second chassiscomprises interconnecting the chassis so as to define a fat tree. 21.The method according to claim 19, and comprising coupling the externalinterconnects of the slots in the second outer tier of the secondswitching chassis to other components of the network, selected from aset of components consisting of other switching chassis and computingnodes.